1. Field of the Invention
The present invention relates to an electric propulsion system for use in electric vehicles. More particularly, the present invention relates to a three-phase power bridge assembly used to drive an electric motor in an electric vehicle propulsion system.
2. Description of the Related Art
Alternating current (AC) electric motors typically contain stator windings and a rotor. AC current is applied to the stator and produces a magnetic field which interacts with the rotor to produce a torque. When a direct current (DC) is used to drive an AC motor, the direct current must first be converted to AC current which is transmitted to the motor.
High voltage AC electric motors, such as those used in electric propulsion systems for an electric vehicle, may employ an inverter circuit including specially designed power switches connected in a bridge configuration to produce AC current to drive the stator windings. Semiconductor switches are desirably used in power bridges of inverter circuit because of their high speed, easy electric control, and reliability. However, most common semiconductor switches cannot withstand high voltages.
Bipolar transistors are, in some ways, attractive candidates for use in power bridges because of their ability to withstand high voltages. High voltage bipolar transistors, however, also require high driving currents, a feature which is undesirable because ordinary metal on silicon field effect transistors (MOSFETs) cannot be used to drive these high voltage bipolar transistors. Therefore, field effect transistors (FETs), which can be driven by MOSFET inputs have been used in power bridges. Unfortunately, single FETs for high voltage operations are extremely expensive, and several hundred lower voltage FETs must be used together in a power bridge. This increases complication, expense, and manufacturing time.
Another problem encountered in power bridges for electric vehicle propulsion system inverter circuits is that rapidly switching a high voltage can create undesirable electromagnetic interference (EMI) which interferes with surrounding electrical components. EMI can be reduced by known filter circuitry using inductors and capacitors. However, interactions of filter capacitors and stray inductances formed in circuit interconnectors can interact to generate undesirable parasitic oscillation currents. For example, electrically conducting brackets commonly used to connect inverter switches, filter capacitors, and a DC input power bus bar can form inductances which give rise to undesirable voltage spikes or parasitic oscillation current. In a high voltage system such as an electric propulsion system for an electric vehicle, such parasitic oscillation currents can interfere with other components of the system.
The mounting brackets are typically soldered to the output terminals of a filter capacitor. These solder joints create an area of thermal stress which may lead to breakdowns of the power bridge. Additionally, soldered joints add manufacturing complication and expense.